Conventionally, ion detectors including a conversion dynode and a secondary-electron multiplier have been used for detecting ions with high sensitivity in a mass spectrometer. In such an ion detector, a high voltage (up to approximately ±10 kV for example) having a polarity opposite to that of the ions to be analyzed is required to be applied to a conversion dynode for selectively detecting positive ions and negative ions. In a liquid chromatograph mass spectrometer, an ion source provided by an electrospray ionization (ESI) method is used for example for vaporizing and simultaneously ionizing a liquid sample. In such an ion source, a high voltage (approximately ± several kV for example) with a polarity equal to that of the ions to be analyzed is required to be applied to the tip of a nozzle for spraying a liquid sample.
In such a use as described earlier, the polarity of the high voltage applied is required to be changed corresponding to whether the object to be analyzed is a positive ion or a negative ion. Therefore, a high-voltage power unit capable of switching the polarity of an output voltage of single system is used. One of the most conventionally well-known high-voltage power units for switching high voltages of different polarity is one using a high-voltage reed relay (See Patent Document 1 for example.).
In a high-voltage power unit with such a configuration, when changing a relay, spike discharges may occur to break the relay. Therefore, when switching the polarity of an output voltage, it is necessary to observe the following procedure: decrease the output voltage, actuate the relay to change the contacts when the output voltage becomes adequately small, and after that, increase the output voltage. Consequently, it takes some time to change the polarity, which elongates the non-detection period. This affects the accuracy of an analysis in the case where, for example, the detection of the positive ions and negative ions are alternately performed by switching them within a short period of time in a mass spectrometer. In addition, a high-voltage relay of 10 kV or above is generally expensive but nonetheless does not have a very long life time. That is, it is costly and also has a problem in reliability.
On the other hand, a high-voltage semiconductor switch (e.g. power MOSFET or the like) might be used in place of a reed switch. The switching operation of a semiconductor switch itself is much faster than that of a relay. However, although a high-voltage semiconductor switch of 10 kV or above is available, it is highly expensive. Even in cases where relatively-inexpensive semiconductor switches are used, since they have to be connected in series in multiple stages, the cost eventually becomes significantly high. And a semiconductor switch also has a problem in reliability since it is weak in the discharge of a high voltage.
[Patent Document 1] U.S. Pat. No. 6,002,600 (refer to FIG. 1C)